Conventionally, laser mirrors have been constructed from multiple layers of two or more dielectric materials having different refractive indices. Each layer is very thin, i.e. having an optical thickness (physical thickness times the refractive index of the layer) on the order of order of ¼ wavelength the light to be reflected in the application. The layers may be deposited on a substrate to provide a high reflectance region extending over a limited range of wavelengths.
It has been recognized that dielectric mirrors provide both a higher reflectance and higher laser damage threshold than more conventional metal mirrors. A major disadvantage associated with conventional dielectric mirrors, however, is that they do not have sufficiently broadband reflectance zones to allow use in connection with the range of laser wavelengths used in certain optical systems. As such, conventional dielectric mirrors must be changed each time a laser with a wavelength outside of the mirror's reflectance zone is used, or to accommodate use of the entire range of a multi-wavelength or tunable device such as a Nd:YAG, Ti-Sapphire, or a dye laser.
It is well known, for example, that an Nd: YAG laser has fundamental and harmonic wavelengths of 1064 nm, 532 nm, and 0.355 nm. Conventionally, it has not been possible to provide a single dielectric mirror that provides high reflectivity of all polarization states in this range of fundamental and harmonic frequencies, plus all wavelengths in between. Thus, separate mirrors have been necessary depending on the selected laser wavelength. Also, many Ti:Sapphire lasers are utilized in “two-photon” fluorescence measurement systems, in which the laser excites a fluorescent sample at a wavelength between about 700 and 1100 nm, and the fluorescence at a wavelength just longer than half the excitation wavelength is measured. Because of the low light levels involved, the sensitivity of such a system is severely limited by the difficulty of attaining a single mirror that directs both the excitation and the fluorescence light.
Attempts have been made to increase the width of the high reflectance zone in a dielectric mirror through use of a chirped stack. In a chirped stack, the thicknesses of the layers in the stack are gradually increased so that the summation of the reflections from the individual interfaces adds up to a large reflectance for a wide range of wavelengths. Despite use of chirped stacks, deposition processes used for depositing the thin-film layers have provided practical limitations to producing mirrors with high reflectance over a broad range of wavelengths.
In a typical deposition process, the surface roughness of the individual layers increases with layer count to the point where the reflection decreases to well below 99%. In addition, random thickness errors that occur during deposition can also reduce reflectance over a portion of the spectral region. Conventionally, layer thickness errors have been reduced through optical monitoring. However, mirrors having high reflectance/low transmittance over a wide range of wavelengths make optical monitoring in conventional deposition systems difficult.
Accordingly, there is a need for a mirror having a high reflectance for all polarization states over a wavelength range spanning the operating wavelengths of a variety of conventional lasers. There is also a need for a mirror having such properties over a wide angle of incidence.